1. Field of the Invention
The present invention relates to a resistance welding method and a resistance welding apparatus for resistance-welding a stacked assembly made up of three or more workpieces, including a thinnest workpiece which is disposed on an outermost side of the stacked assembly.
2. Description of the Related Art
One known process for joining a plurality of metal sheets to each other is a resistance welding process, in which the metal sheets are stacked in a stacked assembly. Then, after the stacked assembly has been gripped and pressed by a set of welding electrodes, an electric current is passed between the welding electrodes in order to melt a region of the metal sheets near contact surfaces thereof. When solidified, the melted region is turned into a solid phase called a nugget. In some cases, three or more metal sheets are joined together by such a resistance welding process.
The thicknesses of metal sheets or workpieces to be resistance-welded are not necessarily identical to each other, and in most cases are different from each other. Therefore, the metal sheets tend to include a workpiece having a smallest thickness (hereinafter also referred to as a “thinnest workpiece”).
If the stacked assembly of metal sheets is resistance welded with the thinnest workpiece disposed on an outermost side of the stacked assembly, then the nugget, which is formed between the thinnest workpiece and a workpiece adjacent thereto, may not grow sufficiently. The reason for insufficient growth of the nugget is considered to be based on the fact that since the thickness of the thinnest workpiece is the smallest and its specific resistance is minimum, the thinnest workpiece fails to generate a sufficient amount of Joule heat.
In order to cause the nugget near the thinnest workpiece to grow sufficiently, Joule heat generated in the thinnest workpiece may be increased by increasing the electric current that is passed between the welding electrodes. However, in this case, large electric current tends to flow through a workpiece of a larger thickness, and thus, such an increased electric current tends to unduly melt the workpiece, thus producing scattering of metal particles and resulting in sputtering.
Alternatively, a period of time during which electric current is applied may be increased. However, in this case also, it is difficult to generate sufficient Joule heat in the thinnest workpiece. Additionally, a longer welding time results in a lower welding efficiency, disadvantageously.
In view of the above difficulties, Japanese Laid-Open Patent Publication No. 2005-262259 proposes a two-stage resistance spot welding process for resistance-welding a stacked assembly made up of three or more metal sheets, with a thinnest workpiece disposed on an outermost side of the stacked assembly. The two-stage resistance spot welding process comprises a first stage in which the pressing force on the stacked assembly is reduced and a large electric current is passed through the stacked assembly, and a second stage in which the pressing force is set to a level greater than the pressing force in the first stage, and during which an electric current equal to or smaller than the electric current in the first stage is passed through the stacked assembly for a time longer than the electric current in the first stage.
According to Japanese Laid-Open Patent Publication No. 2005-262259, the disclosed resistance spot welding process is effective to easily produce a spot-welded joint having a nugget of required size, without adding extra process steps and without causing sputtering.
Nevertheless, it has still been desired in the art to increase the bonding strength of a welded joint through a simpler control process than the process disclosed in Japanese Laid-Open Patent Publication No. 2005-262259.